This invention relates to a dispersant for a pigment-based ink jet ink.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Molecules of the carrier surround each individual dye molecule. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor lightfastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigment-based inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process. Pigment-based inks require a water-soluble dispersant in the pigment slurry during the milling process. Such a dispersant is necessary to produce a stable colloidal mixture and an ink that can be xe2x80x9cjettedxe2x80x9d reliably without clogging the print head nozzles.
Dispersing agents in an ink jet ink have the dual function of helping to break down pigments to sub-micron size during the milling process and of keeping the colloidal dispersion stable and free from flocculation for a long period of time. In general, dispersions suffer from poor colloidal stability due to particle agglomeration and/or sedimentation, thus limiting the usefulness of the dispersions in inks.
Many dispersing agents are very selective in their ability to disperse these pigments to sub-micron size. This is probably one of the many reasons for the wide variety of known dispersing agents. In many instances, each class of pigments may require a specific dispersing agent. Another problem encountered with some dispersing agents is that the resulting inks suffer from unacceptable image quality properties. This can include unacceptable coalescence, banding, bleeding, densities, or other defects seen on the final printed image. Yet another problem with many commercially available dispersants is that they yield dispersions with very low surface tensions. Thus, there is a continuing need for improved dispersing agents for pigment-based inks.
German patent application DE 19850152 discloses the use of ethoxylated alkyl-phenol surfactants for use in the preparation of pigment-based inks for ink-jet printing. However, there is a problem with these dispersants in that pigment dispersions using them have an unacceptably large particle size and are unstable at high temperatures, as will be shown hereinafter.
German patent application DE 19801759 discloses the use of sulfonated aromatic derivatives as dispersants for a wide range of pigment preparations including those in ink-jet printing. However, these dispersants differ significantly in structure to the materials employed in the invention.
It is an object of this invention to provide a dispersant for a pigment-based ink jet ink which will enable the particle size of the dispersed pigment to be lower than that obtained using prior art dispersants disclosed for such use. It is another object of this invention to maintain such particle size for the pigment when the dispersion is held at high temperatures. It is another object of this invention to provide a dispersant for pigment-based ink-jet inks which will produce images which have a high D-max and which are free of defects.
These and other objects are achieved in accordance with this invention which relates to an ink jet ink composition comprising from about 0.5% to about 30% by weight of a pigment, a carrier and a dispersant, the dispersant comprising an anionic derivative of an ethoxylated alkylphenol-formaldehyde resin, the ratio of dispersant:pigment being from about 0.1:1 to about 5:1.
Use of the dispersants in the invention will produce pigment dispersions on the order of 0.1 xcexcm or less in size during the milling process. These dispersions are also characterized by excellent colloidal stability and lack of flocculation and/or sedimentation on keeping at high temperatures. In addition, these dispersing agents are useful with a wide variety of pigments.
Any anionic derivative of an ethoxylated alkylphenol-formaldehyde resin dispersant may be used in the invention provided it produces the desired results. In a preferred embodiment of the invention, the dispersant has the formula: 
wherein: each R independently represents a linear or branched alkyl, alkenyl, or arylalkyl group having from about 4 to about 30 carbon atoms;
x is an integer from 0 to 20;
y is an integer from 1 to 20; with the proviso that x+y is from 2 to 20;
z is an integer from 4 to 20;
A is an anionic group, such as sulfate, sulfonate, phosphate, phosphonate or carboxylate; and
M is a cationic group, such as an alkali metal, alkaline earth metal, ammonium or substituted ammonium.
In a preferred embodiment of the invention, each R independently represents a linear or branched alkyl, alkenyl, or arylalkyl group having from about 6 to 15 carbon atoms, preferably from about 8 to about 12 carbon atoms. In another preferred embodiment, x+y is from 4 to 12, preferably 6 to 8. In still another preferred embodiment, z is an integer from 6 to 14, preferably from 8 to 12.
The ethoxylated alkylphenol-formaldehyde resin dispersant of the invention is a comb-like material which possesses multiple hydrophobic blocks and multiple hydrophilic blocks.
In the present invention, any of the known pigments can be used. Pigments can be selected from those disclosed, for example, in U.S. Pat. Nos. 5,026,427; 5,085,698; 5,141,556; 5,160,370 and 5,169,436, the disclosures of which are hereby incorporated by reference. The exact choice of pigment will depend upon the specific color reproduction and image stability requirements of the printer and application. For four-color printers, a combination of cyan, magenta, yellow, and black (CMYK) pigments should be used. An exemplary four color set is copper phthalocyanine (pigment blue 15), quinacridone magenta (pigment red 122), pigment yellow 74 or pigment yellow 155 and carbon black (pigment black 7). Another exemplary four color set is bis(phthalocyanyl-alumino)tetra-phenyldisiloxane cyan pigment, quinacridone magenta (pigment red 122), pigment yellow 74 or pigment yellow 155 and carbon black (pigment black 7). In a preferred embodiment of the invention, the pigment has a particle size of from about 10 nanometers to about 1000 nanometers.
As noted above, the ink jet ink composition of the invention comprises from about 0.5% to about 30% by weight of a pigment. In a preferred embodiment of the invention, the ink composition comprises from about 1% to about 5% by weight of the pigment.
The carrier employed in the invention can be, for example, water or a mixture of water and at least one water soluble co-solvent. Selection of a suitable mixture depends on requirements of the specific application, such as desired surface tension and viscosity, the selected pigment, drying time of the pigmented ink jet ink, and the type of paper onto which the ink will be printed. Representative examples of water-soluble co-solvents that may be selected include (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; (3) ethers, such as tetrahydrofuran and dioxane; (4) esters, such as ethyl acetate, ethyl lactate, ethylene carbonate and propylene carbonate; (5) polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol 1,2,6-hexanetriol and thioglycol; (6) lower alkyl mono- or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl (or -ethyl) ether, diethylene glycol mono-methyl (or -ethyl) ether, propylene glycol mono-methyl (or -ethyl) ether, triethylene glycol mono-methyl (or -ethyl) ether and diethylene glycol di-methyl (or -ethyl) ether; (7) nitrogen-containing cyclic compounds, such as pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and (8) sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone.
A humectant may also be employed in the ink jet composition of the invention to help prevent the ink from drying out or crusting in the orifices of the printhead. Examples of humectants which can be used include polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol 1,2,6-hexanetriol and thioglycol; lower alkyl mono- or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol mono-methyl or mono-ethyl ether, propylene glycol mono-methyl or mono-ethyl ether, triethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol di-methyl or di-ethyl ether, and diethylene glycol monobutylether; nitrogen-containing cyclic compounds, such as pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone. A preferred humectant for the composition of the invention is diethylene glycol, glycerol, or diethylene glycol monobutylether. The humectant may be present in any amount effective for the intended purpose. In general, good results have been obtained when the humectant is present in an amount of up to about 70% by weight of the ink jet ink composition, preferably from about 5 to about 50%.
Ink Preparation
A preferred method for making the inks of the invention is disclosed in U.S. Pat. No. 5,679,138, the disclosure of which is hereby incorporated by reference. In general it is desirable to make the pigmented ink jet ink in the form of a concentrated mill grind, which is subsequently diluted to the appropriate concentration for use in the ink jet printing system. This technique permits preparation of a greater quantity of pigmented ink from the equipment. If the mill grind was made in a solvent, it is diluted with water and optionally other solvents to the appropriate concentration. If it was made in water, it is diluted with either additional water or water miscible solvents to make a mill grind of the desired concentration. By dilution, the ink is adjusted to the desired viscosity, color, hue, saturation density, and print area coverage for the particular application.
In the case of organic pigments, the ink may contain up to approximately 30% pigment by weight, preferably approximately 1.0 to 5%, by weight of the total ink composition for most thermal ink jet printing applications. The amount of aqueous carrier medium is in the range of approximately 70 to 99.8 weight %, preferably approximately 90 to 99.8 weight %, based on the total weight of the ink. A mixture of water and a polyhydric alcohol, such as diethylene glycol, is preferred as the aqueous carrier medium. In the case of a mixture of water and diethylene glycol, the aqueous carrier medium usually contains from about 30% water/70% diethylene glycol to about 95% water/ 5% diethylene glycol. The preferred ratios are approximately 60% water/40% diethylene glycol to about 95% water/ 5% diethylene glycol. Percentages are based on the total weight of the aqueous carrier medium.
Jet velocity, separation length of the droplets, drop size and stream stability are greatly affected by the surface tension and the viscosity of the ink. Pigmented ink jet inks suitable for use with ink jet printing systems should have a pH in the range of 5 to 9. Anionic and cationic surfactants may be used in the invention as disclosed in U.S. Pat. Nos. 5,324,349; 4,156,616 and 5,279,654 as well as many other surfactants known in the ink jet ink art. Commercial surfactants include the Surfynols(copyright) from Air Products; the Zonyls(copyright) from DuPont Corp. and the Fluorads(copyright) from the 3M Co.
Acceptable viscosity""s are no greater than 20 centipoise, and preferably in the range of about 1.0 to about 10.0, preferably 1.0 to 5.0 centipoise at room temperature.
The ink has physical properties compatible with a wide range of ejecting conditions, i.e., driving voltages and pulse widths for thermal ink jet printing devices, driving frequencies of the piezo element for either a drop-on-demand device or a continuous device, and the shape and size of the nozzle.
Other ingredients are also commonly added to ink jet inks. A penetrant (0-10 wt. %) may be added to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. A preferred penetrant for the inks of the present invention is n-propanol at a final concentration of 1-6 wt. %. A biocide (0.01-1.0 wt. %) may be added to prevent unwanted microbial growth which may occur in the ink over time. A preferred biocide for the inks of the present invention is Proxel(copyright) GXL (Zeneca Colours Co.) at a final concentration of 0.05-0.5 wt. %. Additional additives which may optionally be present in ink jet inks include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, and defoamers.
The ink jet inks provided by this invention are employed in ink jet printing wherein liquid ink drops are applied in a controlled fashion to an ink receptive layer substrate, by ejecting ink droplets from the plurality of nozzles, or orifices, in a print head of ink jet printers
Ink-receptive substrates useful in ink jet printing are well known to those skilled in the art. Representative examples of such substrates are disclosed in U.S. Pat. Nos. 5,605,750; 5,723,211; and 5,789,070 and EP 813 978 A1, the disclosures of which are hereby incorporated by reference.
Commercially available ink jet printers use several different methods to control the deposition of the ink droplets. Such methods are generally of two types: continuous stream and drop-on-demand.
In drop-on-demand systems, a droplet of ink is ejected from an orifice directly to a position on the ink receptive layer by pressure created by, for example, a piezoelectric device, an acoustic device, or a thermal process controlled in accordance digital data signals. An ink droplet is not generated and ejected through the orifices of the print head unless it is needed. Ink jet printing methods, and related printers, are commercially available and need not be described in detail.
The following examples illustrate the utility of the present invention.